1. Field of the Invention
The present invention relates to a sheet processing apparatus that detects a deviation of a sheet in a widthwise direction thereof intersecting a sheet transport direction and correct the deviation, a method for detecting a lateral position deviation amount of a sheet, and an image forming system.
2. Description of the Related Art
A conventional existing sheet processing apparatus includes an apparatus that includes a hole punching mechanism to make a hole in a sheet. Among this type of apparatus, there exists an apparatus in which, when a hole is to be made in a sheet, a deviation amount of a sheet in a widthwise direction thereof intersecting a sheet transport direction (hereinafter, referred to as “lateral position deviation amount”) is corrected in order to improve accuracy in a hole position. Correction of the lateral registration deviation is typically carried out by sensing a lateral position deviation amount while transporting a sheet and by moving the sheet or the hole punching mechanism in the widthwise direction of the sheet intersecting the sheet transport direction, based on the sensing result.
Here, as a method for sensing a lateral position deviation amount of a sheet, there exists a method in which a sheet sensor for sensing the sheet is moved in the widthwise direction of the sheet intersecting the sheet transport direction and the lateral position deviation amount of the sheet is obtained from a moving amount of the sheet sensor from the start of the move until the sheet sensor senses an edge of the sheet in the widthwise direction (see Japanese Laid-Open Patent Publication (Kokai) No. 2005-342943, for example). In such a method, by using a stepping motor as a motor for moving the sheet sensor, the moving amount of the sheet sensor from when the sheet sensor starts moving until the sheet is sensed can be obtained indirectly from a driving pulse of the motor. Accordingly, an apparatus for directly detecting the moving amount of the sheet sensor becomes unnecessary, which makes it possible to detect the lateral position deviation amount of the sheet with an inexpensive configuration.
However, with the above-described conventional technique, it may take time until the sheet sensor comes to move proportionally to a driving amount of a sensor driving motor after the sensor driving motor has started. In particular, in a case where a drive force transmission unit comprises a belt, as the belt stretches, it is apt to take longer for the sheet sensor to come to move proportionally to the driving amount of the sensor driving motor.
Thus, a method of obtaining a moving amount of the sheet sensor from the start of its movement until the sheet is sensed based on the driving pulse of the sensor driving motor has a problem of disabling the moving amount of the sheet sensor to be accurately obtained immediately after the start of the sensor driving motor, thereby reducing sensing precision of the lateral position deviation amount of the sheet with respect to the normal sheet transport position.
Further, to counter such a problem, a stand-by position of the sheet sensor may be away from the sheet so that the sheet sensor does not sense the sheet until the moving amount of the sheet sensor becomes proportional to the driving pulse of the sensor driving motor. However, such a method leads to another problem of the moving amount of the sheet sensor being increased, thereby taking time to sense the lateral position deviation amount of the sheet.